Add in-app help, formula reference outlines, underline and auto-closing

- Help dialog (?, F1, Datei > Hilfe): user topics, all shortcuts and the
  formula functions with one search over everything. Topics and shortcuts
  are plain data in data/help.ts; the function list comes from
  codeReference.ts so new built-ins appear by themselves.
- While a formula is edited, the grid outlines each referenced cell/range
  in the colour of its position in the text, and the outline can be
  dragged to move or resize the reference, as in Excel. Point-mode picks
  (click, drag, arrow keys) are outlined too, so keyboard picking is no
  longer blind.
- Underline as a cell format, plus Mod+Shift+B/I/U toggles that switch on
  unless every selected cell already has the format.
- A formula committed with open parentheses or an open text literal gets
  the missing closers appended; anything that still doesn't parse is left
  alone so the usual error shows instead of a guess.
- Cells and tables with a fill but no text colour get dark or light text
  by WCAG contrast instead of the theme colour (no black on black).

app/frontend/store/format.ts

@@ -165,3 +165,35 @@ export function rawString(v: CellValue): string {
 export function isNumeric(v: CellValue): boolean {
   return typeof v === 'number' && Number.isFinite(v);
 }
+
+// ---- readable text on a fill ---------------------------------------------
+
+const DARK_TEXT = '#1d1d1f';
+const LIGHT_TEXT = '#f5f5f7';
+
+/** sRGB channel (0–255) → linear light, per WCAG 2.x */
+const linear = (c: number) => {
+  const s = c / 255;
+  return s <= 0.03928 ? s / 12.92 : ((s + 0.055) / 1.055) ** 2.4;
+};
+
+/**
+ * Text colour that stays readable on the fill `bg` (#rgb, #rgba, #rrggbb,
+ * #rrggbbaa): dark or light, whichever contrasts more (WCAG luminance).
+ * Used when a cell or table has a fill but no text colour of its own -- the
+ * theme's text colour would otherwise land on it regardless (black text on
+ * a black fill). `null` when `bg` can't be read or is mostly transparent
+ * (the theme's surface shows through, its text colour already fits).
+ */
+export function readableTextOn(bg: string | undefined | null): string | null {
+  const m = /^#([0-9a-f]{3,4}|[0-9a-f]{6}|[0-9a-f]{8})$/i.exec((bg ?? '').trim());
+  if (!m) return null;
+  let hex = m[1];
+  if (hex.length <= 4) hex = [...hex].map((ch) => ch + ch).join('');
+  const [r, g, b] = [0, 2, 4].map((i) => parseInt(hex.slice(i, i + 2), 16));
+  const alpha = hex.length === 8 ? parseInt(hex.slice(6, 8), 16) / 255 : 1;
+  if (alpha < 0.5) return null;
+  const lum = 0.2126 * linear(r) + 0.7152 * linear(g) + 0.0722 * linear(b);
+  // contrast with black vs. with white: (L + 0.05) / 0.05 vs 1.05 / (L + 0.05)
+  return (lum + 0.05) / 0.05 >= 1.05 / (lum + 0.05) ? DARK_TEXT : LIGHT_TEXT;
+}